1. Field of the Invention
The present invention relates to electrodes of electrochemical capacitors and cells, and utilizes simpler one-step radio frequency magnetron sputtering through a dry process to reduce residual impurities in the electrode and then avoid explosion. The present invention also relates to the method of fabricating electrodes for electrochemical capacitors and cells.
2. Description of the Prior Arts
In recent years, electronic industry develops vigorously; finished products tend to be light and small. Demands are eager for stored-energy elements with high power density, high energy density, reversibility and long cycle life. The conventional capacitor mainly uses electric double layers to store charges, and has the characteristic of longer cycle life. For both low energy-storage and short discharge, their applications are limited. The secondary battery could not have important breakthrough even though with quite high energy density, the long use-life (long cycle life), quick charges and high power density are limited by the electrode-surface chemical response. Thus, electrochemical capacitors, also called supercapacitors, are developed, which are a new storage element, and have the characteristics of both secondary lithium battery and the conventional capacitor. Due to high capacitance, the electrochemical capacitor is further used on mixed power sources of electro-vehicles, emergency power supply and memory buffer. Moreover, electrochemical capacitor has the characteristic of storing large amount of energy at a short time, fast charge-discharge, long cycle life, good reversibility, and no contamination, and thus is usually applied in burst-power generation as well as acceleration tool in electric-drive vehicles, and starting power for, notebook as well as fuel cell.
The electrode materials used for electrochemical capacitor currently could be classified to three kinds: active charcoal with high specific surface area, metal oxides and conductive polymeric materials. The capacitor with electrode materials of active carbon with high specific surface area stores energy through physical absorption of charges by electric double layers on electrode surface, but is accompanied by a disadvantage of high internal resistance. The capacitor with electrode materials of metal oxides and conductive polymeric materials stores energy not only by the way as above-mentioned but also by using the charges transference on electrode surface to cause oxidation and reduction to store electrical energy, thus the capacitance of capacitor is higher than that only using the way of electric double layers. The metal oxides usually refer to transition metal oxides such as ruthenium dioxide (RuO2). The transition metal oxides are common electrode material, use of which could generate pseudocapacitance to form a high capacitance. Yet the expensive cost of ruthenium dioxide limits the development of related applications of ruthenium oxides to electrode material. Many studies presently are devoted to other transition metal oxides such as manganese (Mn) oxide, cobalt (Co) oxide, and iron (Fe) oxide to replace ruthenium oxides.
Currently, the manganese oxides are applied to prepare the electrode of electrochemical capacitors in industry to replace ruthenium oxides for reducing costs. However, some residual impurities could be carried over from these wet processes such as sol-gel method or immersion method to prepare manganese metal oxide electrode materials, and these might affect electrochemical properties (such as conductance) of electrodes. During charge-discharge, the accumulating electric currents around impurities would easily produce high heat and then produce small sparks or explosion. Thus, depositing transition metal oxides by a dry process (such as sputtering) is a relatively safe method. Broughton has provided a sputtering method to deposit transition metal (Electrochimica Acta, 49, 4439, 2004). In said process, the thickness of transition metal oxide films could be accurately controlled by depositing manganese, which is oxidized to manganese dioxide through anodic oxidation. The mass specific capacitance of the electrodes with the films is determined to be 400 to 450 F g−1 at a potential scan rate of 5 mV s−1 by cyclic voltammetry. However, said process includes two stages: dry process (sputtering) and wet process (anodic oxidation), and does not test charge-discharge for a long time, and merely tests capacitance under lower potential scan rate (such as 5 mV s−1).
In conclusion, there are many problems in terms of material and process in the electrodes of electrochemical capacitors and cells, which are to be overcome and improved.